DFS client computers (webfarm nodes in this case) cache the DFS information for the length of time that you specify.  This should not be too low or you will have too much traffic to the Namespace server, but it should not be too high or changes to the namespace and failbacks to a restored server will take a long time to be noticed.  It is up to your environment what you want to set this at, but in every situation it is important to know that there is some caching that takes place.

Make sure to keep in mind that adding a new folder to your DFS namespace will not be noticed immediately.  You can force the DFS client cache to be cleared by running dfsutil /PktFlush from the client server.  The dfsutil.exe is a tool that is available in the Windows Server 2003 /support/tools folder of the installation CD.  I simply copy that file to C:\Windows\System32 and I can run dfsutil from the command prompt.

When setting up new sites, make sure to wait until the new site has been recognized by all of the webfarm nodes or force a cache flush from all of the nodes before attempting to set up or update the site.